Preparation of partially cross-linked polymers and their use in pattern formation

ABSTRACT

This invention relates to a process for generating a organically soluble partially cross-linked acid labile polymer according to the present invention comprises the steps of providing a polymer with one or more monomer units, wherein at least one of the monomer units contain one or more pendant COOH or hydroxyl groups; and reacting said polymer with a polyvinyl ether in the presence of a acid catalyst to form links between at least two polymer chains. The resulting polymer can be used as a component in a photoresist formulation.

FIELD OF THE INVENTION

The present invention relates to a process for preparing polymers, whichare used in the production of images. More particularly, it relates tothe preparation of partially cross-linked acid labile polymerscomprising hydroxyl or COOH groups, acid labile protecting groups andacid labile polymer chain linking groups.

BACKGROUND TO THE INVENTION

The wavelength of light for lithography has been reduced into the deepultraviolet (DUV) range to produce the feature size necessary forcurrent and future electronics devices. The electronics industry isdeveloping new resists that are tailored to the DUV range. One suchresist class is chemically amplified resists.

The main components of chemically amplified resist formulations are aphotoacid generator compound, a polymer resin and a solvent capable ofdissolving the photoacid generator and the resin. For many positivechemically amplified resists, the polymer resin contains acid labilegroups which make the polymer resin insoluble in an aqueous developer.Upon irradiation, the photoacid generator compound produces an acidwhich cleaves the acid labile groups resulting in a polymer resin thatis aqueous soluble. Chemically amplified resists have generated a greatdeal of interest and there are numerous patents available discussingthese compositions such as, for example, U.S. Pat. Nos. 5,069,997;5,035,979; 5,670,299; 5,558,978; 5,468,589; and 5,389,494.

One group of polymers which can be used as resins in chemicallyamplified resists are acetal or ketal functionalized polymers. Thealkali solubility of phenolic resins is greatly inhibited by convertingthe hydroxyl groups to acetal or ketal groups. Typically, acetal orketal phenolic resins are produced by reacting a phenolic resin with avinyl ether in the presence of an acid catalyst.

In addition, patents such as U.S. Pat. Nos. 5,670,259 to Imai et al.(Imai) and 5,714,559 to Schacht et al. (Schacht) disclose the use ofcross-linked groups between two polymer chains. In Imai, thecross-linking groups are prepared by coating a substrate with (i) apolymer containing phenol or carboxyl groups and (ii) a compound withtwo to four vinyl ether groups. The substrate is then heated to form thecross-links between the polymer chains.

In Schacht, the acid labile cross-links are formed by reacting a polymercontaining repeating units of hydroxystyrene and a vinylcyclohexanol inthe presence of an acid catalyst to form acetal or ketal cross-links.

The acid-labile polymers are formulated with a photoacid generatorcompound to form a chemically amplified resist product. Uponirradiation, the generated acid cleaves the acid-labile protectinggroups resulting in a photoresist which is soluble in an aqueousdeveloper and thereby enhances the solubility of the polymer in theexposed areas without dissolving the unexposed areas.

The problem with the invention in Imai is that the amount ofcross-linking is very sensitive to the baking conditions. If the bakingconditions are not strictly controlled, the amount of cross-linking willbe not be reproducible. This will change the dissolution and exposurecharacteristics of the resist resulting in a small lithographic processwindow.

In the Schacht patent, cross-linking between two phenol units of twodifferent polymers was shown to be possible using poly(hydroxystyrene),monovinyl ether and an acid. However, the cross-linking between the twopoly(hydroxystyrene) polymer chains is not an efficient and reproducibleprocess in the presence of a monovinyl ether and an acid. Consistentcross-linking is necessary since important thermal and lithographicproperties depend upon the reproducibility and extent of cross-linking.

Schacht also showed that cross-linking was formed between phenolic unitsand alcohol units, (such as cyclohexanol), in the presence of monovinylether and an acid. This mode of cross-linking requires a polymer ofhydroxystyrene and hydroxycyclohexyl vinyl monomers. These type ofcopolymers are produced by partially hydrogenating thepoly(hydroxystyrene) and such hydrogenation does not give reproducibleconcentrations of hydroxy-cyclohexyl moiety in the polymer. Therefore,the variability in the percentage of cyclohexyl moiety can dramaticallyinfluence the extent and reproducibility of cross-linking.

Another problem with acid labile, acetal-based resist is that thevolatile by-products are formed after exposure to actinic radiation.These volatile by-products may coat the lens of the exposure toolresulting in reliability problems. In addition, volatile by-productscause the resist to shrink, which is not desirable for semiconductormanufacturing.

Therefore, it is an object of the present invention to provide a methodof reproducibly preparing a partially cross-linked acid labile polymersuitable for use as a component in a chemically amplified photoresist.The photoresist will have high contrast, increased sensitivity andimproved high temperature flow characteristics.

It is another object of this invention to provide a photoresist that haslow volatility by-products when exposed to radiation, which will reducethe reliability problems of the exposure tool and results in lessshrinkage of the resist.

SUMMARY OF THE INVENTION

The present invention relates to a process for preparing a organicallysoluble partially cross-linked acid-labile polymer. The resultingpartially cross-linked acid-labile polymer may be blended with aphotoacid generator in a solvent to formulate a chemically amplifiedresist composition, which is used in the production of electronicsdevices. The resulting resist composition is highly reproducible and hasa large process window.

The general process for generating an organically soluble partiallycross-linked acid labile polymer according to the present inventioncomprises the steps of providing a polymer with one or more monomerunits, wherein at least one of the monomer units contain one or morependant COOH or hydroxyl groups; and reacting this polymer with apolyvinyl ether in the presence of an acid catalyst to form linksbetween at least two polymer chains. In the present invention, polyvinylether means a compound with two or more vinyl ethers.

In a further embodiment of the present invention, a monovinyl ether isadded to the above process to form a ketal or acetal protecting groupsby functionalizing the monomer units having COOH or hydroxyl pendantgroups. In the present invention, monovinyl ether means a compound withonly one vinyl ether.

One advantage of this process is that the degree of cross-linking isvery reproducible. Unlike U.S. Pat. No. 5,670,259 to Imai, whichrequires a sensitive bake to cross-link the polymer, the presentinvention performs the cross-linking in-situ and can reproduciblycontrol the degree of cross-linking by tightly controlling the amount ofhighly reactive polyvinyl ether used in the reaction. It is alsoreproducible since the highly reactive polyvinyl ether can efficientlyand reproducibly form cross-links between two phenolic units ofpoly(hydroxystyrene) chains. This is in contrast to the Schacht patentwhere the monovinyl ether and the acid do not reproducibly formcross-links between two poly(hydroxystyrene) chains.

The invention also provides that the resulting partially cross-linkedacid labile polymer is blended with a photoacid generator and dissolvedin a solvent to produce a chemically amplified resist composition. Othercomponents to the resist composition can be added such as dyes,surfactants, stabilizers, and the like.

This invention further provides a process for forming a pattern whichcomprises the steps of providing the chemically amplified resistcomposition with the organically soluble partially cross-linked acidlabile polymer, coating a substrate with the resist composition,imagewise exposing the resist coated substrate to actinic radiation, andforming a resist image by developing the resist coated substrate.Further processing of the substrate may take place after the formationof the resist image.

The advantages of using the partially cross-linked acid labile polymerin a resist system is that these polymers have higher glass transitiontemperatures which increase the resistance to flow during hightemperature process conditions. Since the amount of cross-linking can betightly controlled, one can readily achieve the desired thermalproperties of the resulting resist. In addition, cross-linking alsoincrease the molecular weight of the final polymer. The large differencein molecular weight between the exposed and unexposed areas of thepolymer results in greater contrast and improved resolution and profileof the resist. Furthermore, cross-linked polymers tend to have betteradhesion to the wafer after development.

In addition, when the acid labile ketal or acetal protecting groups areutilized with the partially cross-linked polymer, there is an increasein contrast and resolution. The acid labile functionalized units areinsoluble in aqueous solution and thus prevents development in theunexposed regions resulting in more vertical profiles.

Another advantage of the present invention is that the higher carboncontaining acetal or ketal protecting groups have less volatiledecomposition alcohol by-products when the organically soluble partiallycross-linked polymer is exposed to radiation. Thus, since the amount ofvolatile alcohol by-products are reduced, there is less shrinkage of theresist, and thereby less chance of coating the lens of the exposuretool. Also, since the higher carbon containing acetal or ketal groupsalcohol by-products are not volatile and remain in the resist, itbecomes more aqueous soluble. This increases the dissolution rate of theexposed resist, as compared to the unexposed resist, resulting in ahigher contrast and better resolution.

Other and further objects, advantages and features of the presentinvention will be understood by reference to the followingspecification.

DETAILED DESCRIPTION AND EMBODIMENTS

The process for producing the partially cross-linked acid labilepolymers, the photoresist compositions containing the partiallycross-linked acid labile polymers, and the process steps for producingthe resist image in accordance with the present invention are asfollows.

The general process for generating an organically soluble, partiallycross-linked acid labile polymer according to the present inventioncomprises the steps of providing a reactant polymer with one or moremonomer units, wherein at least one of the monomer units contain one ormore pendant COOH or hydroxyl groups; and contacting the reactantpolymer with a polyvinyl ether in the presence of an acid catalyst toform links between at least two polymer chains. The preferred hydroxylbased reactant polymers are phenolic or hydroxycycloalkyl-based polymersor mixtures thereof. The more preferred phenolic-based reactant polymersare polyhydroxystyrene (PHS) and novolaks, and the more preferredhydroxycycloalkyl-based reactant polymer is polyvinylcyclohexanol.

Any suitable polyvinyl ether may be used for this cross-linking process.The preferred polyvinyl ethers have the general formula: ##STR1##wherein Y is a polyvalent radical; R¹, R² and R¹⁹ are each independentlyselected from a hydrogen, a linear or branched C₁ to C₁₈ alkyl orhaloalkyl, a C₃ to C₁₈ cycloalkyl, a C₆ to C₁₄ aromatic, a C₆ to C₃₀alkaryl or a C₆ to C₃₀ aralkyl; and n is an integer of 2 or more.Preferably, Y is a linear or branched C₁ to C₃₀ alkylene, haloalkyleneor oxyalkylene, a C₆ to C₁₀ cycloalkylene or substituted cycloalkylene,a C₆ to C₁₄ aromatic, a C₆ to C₃₀ alkarylene or a C₆ to C₃₀ aralkylene.

The alkyl groups represented by R¹, R² and R¹⁹ include, but are notlimited to, methyl, ethyl, propyl, butyl, amyl, hexyl, heptyl, octyl,nonyl, decyl, undecyl, dodecyl, and the like. The halogens of thehaloalkyl represented by R¹, R² and R¹⁹ include chlorine, bromine,fluorine, and iodine. The aralkyl groups represented by R¹, R² and R¹⁹include, but are not limited to, benzyl, phenethyl, phenylpropyl,methylbenzyl, methylphenethyl and ethylbenzyl. Preferably R¹, R² and R¹⁹are hydrogens.

The alkylene group represented by Y include, but are not limited, tomethylene, ethylene, propylene, butylene, amylene, hexylene, heptylene,octylene, nonylene, decylene, undecylene, dodecylene, and the like. Theoxyalklene groups represented by include oxymethylene, oxyethylene,oxypropylene, oxybutylene, oxyamylene, oxyhexylene, and the like. Thearalkyl groups represented by Y include, but are not limited to,benzylene, phenethylene, phenylpropylene, methylbenzylene,methylphenethylene and ethylbenzylene. The cycloalkylene groupsrepresented by Y include cyclohexylene and substituted cyclohexylene.The preferred Y is: ##STR2##

More preferably, the polyvinyl ether is selected from the groupconsisting of: cyclohexanedimethanol divinyl ether, ethyleneglycoldivinyl ether, butanediol divinyl ether, hexanedimethanol divinyl ether,diethyleneglycol divinyl ether, triethyleneglycol divinyl ether,tetraethyleneglycol divinyl ether, trimethylol propane trivinyl ether,erithritol tetravinyl ether, poly-ethylene oxide divinyl ether andpoly-butylene oxide divinyl ether.

In another embodiment of the present invention, a monovinyl ether isadded to the above described process to functionalize the hydroxyl orCOOH groups and form acetal or ketal protecting groups. Any suitablevinyl ethers may be used for the acetalization or ketalization process.Preferably, the monovinyl ether has the formula: ##STR3## wherein R³, R⁴and R²⁰ are each independently selected from a hydrogen, a linear orbranched C₁ to C₁₈ alkyl or haloalkyl, a C₃ to C₁₈ cycloalkyl, a C₆ toC₁₄ aromatic, a C₆ to C₃₀ alkaryl or a C₆ to C₃₀ aralkyl; and R⁵ is alinear or branched C₁ to C₁₈ alkyl or haloalkyl, a C₃ to C₁₈ cycloalkyl,a C₆ to C₁₄ aromatic, a C₆ to C₃₀ alkaryl, a C₆ to C₃₀ aralkyl or alinear, branched, cyclic, aromatic or olefinic group.

The alkyl groups represented by R³, R⁴ and R²⁰ include, but are notlimited to, methyl, ethyl, propyl, butyl, amyl, hexyl, heptyl, octyl,nonyl, decyl, undecyl, dodecyl, and the like. The halogens of thehaloalkyl represented by R³, R⁴ and R²⁰ include chlorine, bromine,fluorine, and iodine. The aralkyl groups represented by R³, R⁴ and R²⁰include, but are not limited to, benzyl, phenethyl, phenylpropyl,methylbenzyl, methylphenethyl and ethylbenzyl. Preferably, R³, R⁴ andR²⁰ are hydrogens.

The alkyl groups represented by R⁵ include, but are not limited to,methyl, ethyl, propyl, butyl, amyl, hexyl, heptyl, octyl, nonyl, decyl,undecyl, dodecyl, and the like. The halogens of the haloalkylrepresented by R⁵ include chlorine, bromine, fluorine, and iodine. Thearalkyl groups represented by R⁵ include, but are not limited to,benzyl, phenethyl, phenylpropyl, methylbenzyl, methylphenethyl andethylbenzyl. The preferable R⁵ groups are secondary and tertiary alkylspreferably having greater than 6 carbon atoms. The more preferable R⁵group is C₆ to C₃₀ alkyl, aralkyl or alkaryl whose photodecompositionproducts have a boiling point greater than 100C. Higher boiling pointdecomposition by-products result in less volatility, less resistshrinkage and higher contrast as was described above.

The more preferred vinyl ethers are ethyl vinyl ether, tertiary-butylvinyl ether cyclohexyl vinyl ether, 2-ethyl hexyl vinyl ether anddodecyl vinyl ether.

Other suitable reactant polymers for this process are novolaks which aretypically used as resins for photoresist. The hydroxyl sites of novolaksmay be also be functionalized with the present process to form acidlabile protecting or cross-links groups.

It should also be noted that each repeating unit of the polymer maycontain one or more hydroxyl or COOH groups. For example, the reactantpolymer may contain a dihydroxy phenyl repeating unit. The acid labilefunctionalization reaction may occur on none, either, or both hydroxylsites depending on the overall degree of functionalization.

The overall degree of functionalization and derivatization of thehydroxyl or COOH sites is controlled by the amount of monovinyl ethersand polyvinyl ether respectively used in the feedstock. The degree ofcross-linking will increase as the amount of polyvinyl ether isincreased; while the degree of acetalization or ketalization willincrease as the amount of monovinyl ether is increased.

It should be noted that the order of reaction can be performed in anysuitable manner. For example, the reactant polymer can be acetal orketal functionalized before the resulting polymer is cross-linkedutilizing the polyvinyl ethers. On the other hand, the reactant polymercan first be cross-linked before the acetal or ketal protecting reactionwith the monovinyl ether. Preferably, the acid catalyst, monovinyl etherand polyvinyl ether are added together so that only one synthesis isrequired.

In a typical synthesis procedure, a hydroxyl or COOH based reactantpolymer or copolymer is dissolved in any suitable solvent or solventmixture. The solvent present should be inert under the reactionconditions. Suitable solvents may include aromatic hydrocarbons,chlorinated hydrocarbons, esters, and ethers such as tetrahydrofuran,(THF), 1,4-dioxane, methylene chloride, propylene glycol monomethylether acetate, (PGMEA), and dimethoxyethane. Preferred solvents for thereaction are THF and PGMEA.

To such a polymer solution mentioned above, the polyvinyl and monovinylethers are added at room temperature. The desired concentration ofreactant polymer or copolymer dissolved in the solvent is about 10weight percent to 60 weight percent. The amounts of polyvinyl ether willvary from about 0.0001 to 5 mole % of the total moles of COOH orhydroxyl groups in the polymer, while the monovinyl ether may vary fromabout 0.01 mole percent to 60 mole percent of the total moles of COOH orhydroxyl groups. The preferable range of polyvinyl ether is 0.1 to 1.5%mole percent; while the monovinyl ether is about 5 mole percent to 40mole percent.

An acid catalyst is added and the reaction mixture is allowed to stirfor about 4 to 24 hours. The preferred reaction time is about 20 hours.Any suitable acid catalyst may be used for the reaction such ashydrochloric acid, sulfuric acid, malonic acid, oxalic acid,para-toluene sulfonic acid and pyridinium-para-toluene sulfonate. Thepreferred acid catalyst is oxalic acid. The acid catalyst may be addedin amounts ranging from about 0.0001 weight percent to 3.0 weightpercent based on the weight of the polymer. The preferred amount of acidcatalyst added is about 0.001 to 0.1 weight percent. The acid catalystis normally quenched with an organic or inorganic base. The acid labilederived hydroxystyrene based polymer is isolated by any suitable polymerisolation procedure such as by precipitation in a non-solvent.

In a preferred embodiment, the hydroxyl based reactant polymer has theformula: ##STR4## wherein X is defined as: ##STR5## or mixtures thereof;R⁶, R⁸ and R⁹ are each independently a hydrogen, a C₁ to C₄ alkyl, ahalogen, a nitro, a cyano or combinations thereof.

Preferred reactant polymers are hydroxyl based polymers such aspolyhydroxystyrene, novolaks or polyvinylcyclohexanol and mixturesthereof.

In an another embodiment of the invention, the structure of the backbonein the reactant polymer may be further modified to include aqueousinsoluble monomers such as: ##STR6## wherein R¹⁰ is a hydrogen, a C₁ toC₄ alkyl, a halogen, a nitro, a cyano or a combination thereof; R¹¹ is ahydrogen, a linear of branched C₁ to C₁₈ alkyl or haloalkyl, a C₃ to C₁₈cycloalkyl, a C₆ to C₁₄ aromatic, a C₆ to C₃₀ alkaryl or a C₆ to C₃₀aralkyl; R¹² is a hydrogen, methyl or ethyl group, or a group having theformula --CH₂ --COOR⁷ ; R⁷ is a primary, secondary and tertiary carbonattached to an alkyl or aromatic group; R¹³ is a valent bond ormethylene; R¹⁴ is a primary, secondary and tertiary carbon attached toan alkyl or aromatic group and X¹ is defined as ##STR7##

In a further embodiment of the invention, the reactant polymer, such aspolyhydroxystyrene or polyvinylcyclohexanol-based polymers may furtherbe modified to incorporate a tertiary butoxycarbonyloxy (t-BOC) ortertiary-butyloxycarbonylmethoxy (BOCMe) functional groups. The t-BOCfunctional group can be introduced by reacting the hydroxyl containingpolymers and copolymers with di-tertiary-butyl dicarbonate in thepresence of any suitable organic or inorganic base such as dimethylamino pyridine. Similarly, BOCMe functional group can be introduced byreacting the polymers or copolymers with tertiary-butyl bromoacetate.With these reactions, for example, one of the monomer units of thefunctionalized polymer of hydroxystyrene would be as follows: ##STR8##wherein R⁶ is as defined above; and R¹⁶ is t-BOC or BOCMe.

Thus, the alkali insoluble monomer unit may contain either acidsensitive or non-acid sensitive groups. Preferred monomer units withacid sensitive groups include t-butoxystyrene, tertiarybutoxycarbonyloxy (t-BOC) styrene, acetal protected hydroxystyrene, ortertiary-butyloxycarbonyl-methyl (BOCMe) protected hydroxystyrene,and/or acid sensitive (meth)acrylates such as t-butyl (meth)acrylate.Preferred non-acid sensitive monomers include various (meth)acrylates,and substituted or unsubstituted styrenes (e.g., styrene,4-acetoxystyrene).

In another preferred embodiment, the reactant polymer will have thefollowing repeating monomer units: ##STR9## with an optional repeatingunit having either formula set for below:: ##STR10##

After the polyvinyl ether, acid catalyst, monovinyl ethers, and,optional, di-tertiary-butyl dicarbonate and/or tertiary-butylbromoacetate are reacted with the reactant polymer with, for example,all three monomer repeating units above, the resulting organicallysoluble partially cross-linked polymer would have the following monomerrepeating units: ##STR11## and at least one monomer unit selected fromthe formulae: ##STR12## wherein X, X¹, R¹ to R²⁰ are defined above, andk is an integer of 1 or more.

The more preferred organically soluble cross-linked polymer has therepeating units I, II, III and VI.

Preferred mole % concentration for the repeating monomer units of theorganically soluble, cross-linked acid labile polymer are shown in Table1 below:

                  TABLE I                                                         ______________________________________                                        Monomer                                                                       Unit   Preferred Mole % Range                                                                        More Preferred Mole % Range                            ______________________________________                                        I      0.001 to 3%     0.1 to 1.5%                                            II     40 to 95%       50 to 90%                                              III    10 to 50%       10 to 35%                                              IV     0 to 40%         0 to 25%                                              V      0 to 40%         0 to 10%                                              VI     0 to 20%         5 to 15%                                              ______________________________________                                    

The invention further relates to the formulation of photoresistcompositions comprising a partially cross-linked acid labile polymer asproduced above, a photoacid generator and a solvent capable ofdissolving both the partially cross-linked acid labile polymer and thephotoacid generator. The preferred partially-cross-linked acid labilepolymers for the photoresist compositions are those that were previouslydescribed in the preferred partially cross-linked acid labile polymerprocess and composition embodiments above.

As described previously, the organically soluble partially cross-linkedpolymer of the present invention has many advantages when used in aphotoresist. The advantages of using the partially cross-linked acidlabile polymer in a resist system is that these polymers have higherglass transition temperatures which increase the resistance to flowduring high temperature process conditions. Since the amount ofcross-linking can be tightly controlled, one can achieve the desiredthermal properties of the resulting resist. In addition, cross-linkingalso increase the molecular weight of the final polymer. The largedifference in molecular weight between the exposed and unexposed areasof the polymer results in greater contrast and improved resolution andprofile of the resist. Furthermore, cross-linked polymers tend to havebetter adhesion to the wafer after development.

In addition, when the acid labile ketal or acetal protecting groups areutilized with the partially cross-linked polymer, there is an increasein contrast and resolution. The acid labile functionalized units areinsoluble in aqueous solution and thus prevent development in theunexposed regions resulting in more vertical profiles.

It is also believed that the higher carbon (6 or more carbons)containing acetal or ketal protecting groups generate less volatilealcohol by-products upon exposure to radiation. Volatile alcoholby-products can coat the lens of the exposure tool resulting inincreased exposure tool reliability problems. Also, the non-volatilealcohol by-products, generated by the higher carbon-containing acetal orketal groups remain in the resist, which reduces resist shrinkage andenhances aqueous solubility. This increases the dissolution rate of theexposed resist as compared to the unexposed resist resulting in a highercontrast and better resolution.

Any suitable photoacid generator compounds may be used in thephotoresist composition. The photoacid generator compounds are wellknown and include, for example, onium salts such as diazonium,sulfonium, sulfoxonium and iodonium salts, and disulfones. Suitablephotoacid generator compounds are disclosed, for example, in U.S. Pat.Nos. 5,558,978 and 5,468,589 which are incorporated herein by reference.

Suitable examples of photoacid generators are phenacylp-methylbenzenesulfonate, benzoin p-toluenesulfonate,α-(p-toluenesulfonyloxy)methylbenzoin3-(p-toluenesulfonyloxy)-2-hydroxy-2-phenyl-1-phenylpropyl ether,N-(p-dodecylbenzenesulfonyloxy)-1,8-naphthalimide andN-(phenylsulfonyloxy)-1,8-napthalimide.

Other suitable compounds are o-nitrobenzaldehydes which rearrange onactinic irradiation to give o-nitrosobenzoic acids such as 1-nitrobenzaldehyde and 2,6-nitrobenzaldehyde, α-haloacylphenones such asα,α,α-trichloroacetophenone andp-tert-butyl-α,α,α-trichloroacetophenone, and sulfonic esters ofo-hydroxyacylphenones, such as 2-hydroxybenzophenone methanesulfonateand 2,4-hydroxybenzophenone bis(methanesulfonate).

Still other suitable examples of photoacid generators aretriphenylsulfonium bromide, triphenylsulfonium chloride,triphenylsulfonium iodide, triphenylsulfonium hexafluorophosphate,triphenylsulfonium hexafluoroarsenate, triphenylsulfoniumhexafluoroarsenate, triphenylsulfonium trifluoromethanesulfonate,diphenylethylsulfonium chloride, phenacyldimethylsulfonium chloride,phenacyltetrahydrothiophenium chloride,4-nitrophenacyltetrahydrothiopheniumn chloride and4-hydroxy-2-methylphenylhexahydrothiopyrylium chloride.

Further examples of suitable photoacid generators for use in thisinvention are bis(p-toluenesulfonyl)diazomethane, methylsulfonylp-toluenesulfonyldiazomethane,1-cyclo-hexylsulfonyl-1-(1,1-dimethylethylsulfonyl)diazometane,bis(1,1-dimethylethylsulfonyl)diazomethane,bis(1-methylethylsulfonyl)diazomethane,bis(cyclohexylsulfonyl)diazomethane,1-p-toluenesulfonyl-1-cyclohexylcarbonyldiazomethane,2-methyl-2-(p-toluenesulfonyl)propiophenone,2-methanesulfonyl-2-methyl-(4-methylthiopropiophenone, 2,4-methyl-2-(p-toluenesulfonyl)pent-3-one,1-diazo-1-methylsulfonyl-4-phenyl-2-butanone,2-(cyclohexylcarbonyl-2-(p-toluenesulfonyl)propane,1-cyclohexylsulfonyl-1cyclohexylcarbonyldiazomethane,1-diazo-1-cyclohexylsulfonyl-3,3-dimethyl-2-butanone,1-diazo-1-(1,1-dimethylethylsulfonyl)-3,3-dimethyl-2-butanone,1-acetyl-1-(1-methylethylsulfonyl)diazomethane,1-diazo-1-(p-toluenesulfonyl)-3,3-dimethyl-2-butanone,1-diazo-1-benzenesulfonyl-3,3-dimethyl-2-butanone,1-diazo-1-(p-toluenesulfonyl)-3-methyl-2-butanone, cyclohexyl2-diazo-2-(p-toluenesulfonyl)acetate, tert-butyl2-diazo-2-benzenesulfonylacetate,isopropyl-2-diazo-2-methanesulfonylacetate, cyclohexyl2-diazo-2-benzenesulfonylacetate, tert-butyl 2diazo-2-(p-toluenesulfonyl)acetate, 2-nitrobenzyl p-toluenesulfonate,2,6-dinitrobenzyl p-toluenesulfonate, 2,4-dinitrobenzylp-trifluoromethylbenzenesulfonate.

Other suitable examples of photogenerators arehexafluorotetrabromo-bisphenol A,1,1,1-tris(3,5-dibromo-4-hydroxyphenyl)ethane andN-(2,4,6-tribromophenyl)-N'-(p-toluenesulfonyl)urea.

The photoacid generator compound is typically employed in the amounts ofabout 0.0001 to 20% by weight of polymer solids and more preferablyabout 1% to 10% by weight of polymer solids.

The choice of solvent for the photoresist composition and theconcentration thereof depends principally on the type of functionalitiesincorporated in the acid labile polymer, the photoacid generator, andthe coating method. The solvent should be inert, should dissolve all thecomponents in the photoresist, should not undergo any chemical reactionwith the components and should be re-removable on drying after coating.Suitable solvents for the photoresist composition may include ketones,ethers and esters, such as methyl ethyl ketone, methyl isobutyl ketone,2-heptanone, cyclopentanone, cyclehexanone, 2-methoxy-1-propyleneacetate, 2-methoxyethanol, 2-ethoxyothanol, 2-ethoxyethyl acetate,1-methoxy-2-propyl acetate, 1,2-dimethoxy ethane ethyl acetate,cellosolve acetate, propylene glycol monoethyl ether acetate, methyllactate, ethyl lactate, methyl pyruvate, ethyl pyruvate, methyl3-methoxypropionate, ethyl 3-methoxypropionate, N-methyl-2-pyrrolidone,1,4-dioxane, ethylene glycol monoisopropyl ether, diethylene glycolmonoethyl ether, diethylene glycol monomethyl ether, diethylene glycoldimethyl ether, and the like.

In an additional embodiment, base additives may be added to thephotoresist composition. The purpose of the base additive is to scavengeprotons present in the photoresist prior to being irradiated by theactinic radiation. The base prevents attack and cleavage of the acidlabile groups by the undesirable acids, thereby increasing theperformance and stability of the resist. The percentage of base in thecomposition should be significantly lower than the photoacid generatorbecause it would not be desirable for the base to interfere with thecleavage of the acid labile groups after the photoresist composition isirradiated. The preferred range of the base compounds, when present, isabout 3% to 50% by weight of the photoacid generator compound. Suitableexamples of base additives are 2-methylimidazole, triisopropylamine,4-dimethylaminopryidine, 4,4'-diaminodiphenyl ether, 2,4,5 triphenylimidazole and 1,5-diazobicyclo[4.3.0]non-5-ene.

Dyes may be added to the photoresist to increase the absorption of thecomposition to the actinic radiation wavelength. The dye must not poisonthe composition and must be capable of withstanding the processconditions including any thermal treatments. Examples of suitable dyesare fluorenone derivatives, anthracene derivatives or pyrenederivatives. Other specific dyes that are suitable for photoresistcompositions are described in U.S. Pat. No. 5,593,812.

The photoresist composition may further comprise conventional additivessuch as adhesion promoters and surfactants. A person skilled in the artwill be able to choose the appropriate desired additive and itsconcentration.

The invention further relates to a process for forming a pattern on asubstrate which comprises the following process steps: application of aphotoresist coating comprising one of the compositions described aboveto the substrate; imagewise exposure of the coating to actinicradiation; treatment of the coating with an alkaline aqueous developeruntil the areas of the coating which have been exposed to the radiationdetach from the substrate and an imaged photoresist structured coatingremains on the substrate.

The photoresist composition is applied uniformly to a substrate by knowncoating methods. For example, the coatings may be applied byspin-coating, dipping, knife coating, lamination, brushing, spraying,and reverse-roll coating. The coating thickness range generally coversvalues of about 0.1 to more than 10 μm. After the coating operation, thesolvent is generally removed by drying. The drying step is typically aheating step called soft bake where the resist and substrate are heatedto a temperature of about 50° C. to 150° C. for about a few seconds to afew minutes; preferably for about 5 seconds to 30 minutes depending onthe thickness, the heating element and end use of the resist.

The photoresist compositions are suitable for a number of different usesin the electronics industry. For example, it can be used aselectroplating resist, plasma etch resist, solder resist, resist for theproduction of printing plates, resist for chemical milling or resist inthe production of integrated circuits. The possible coatings andprocessing conditions of the coated substrates differ accordingly.

For the production of relief structures, the substrate coated with thephotoresist composition is exposed imagewise. The term `imagewise`exposure includes both exposure through a photomask containing apredetermined pattern, exposure by means of a computer controlled laserbeam which is moved over the surface of the coated substrate, exposureby means of computer-controlled electron beams, and exposure by means ofX-rays or UV rays through a corresponding mask.

Radiation sources which can be used are all sources which emit radiationin which the photoacid generator is sensitive. Examples are argon ion,krypton ion, electron beams and x-rays sources.

The process described above for the production of relief structurespreferably comprises, as a further process measure, heating of thecoating between exposure and treatment with the developer. With the aidof this heat treatment, known as "post-exposure bake", virtuallycomplete reaction of the acid labile groups in the polymer resin withthe acid generated by the exposure is achieved. The duration andtemperature of this post-exposure bake can vary within broad limits anddepend essentially on the functionalities of the polymer resin, the typeof acid generator and on the concentration of these two components. Theexposed resist is typically subjected to temperatures of about 50° C. to150° C. for a few seconds to a few minutes. The preferred post exposurebake is from about 80° C. to 130° C. for about 5 seconds to 300 seconds

After imagewise exposure and any heat treatment of the material, theexposed areas of the photoresist are removed by dissolution in adeveloper. The choice of the particular developer depends on the type ofphotoresist; in particular on the nature of the polymer resin or thephotolysis products generated. The developer can comprise aqueoussolutions of bases to which organic solvents or mixtures thereof mayhave been added. Particularly preferred developers are aqueous alkalinesolutions. These include, for example, aqueous solutions of alkali metalsilicates, phosphates, hydroxides and carbonates, but in particular oftetra alkylammonium hydroxides, and more preferably tetramethylammoniumhydroxide (TMAH). If desired, relatively small amounts of wetting agentsand/or organic solvents can also be added to these solutions.

After the development step, the substrate carrying the resist coating isgenerally subjected to at least one further treatment step which changessubstrate in areas not covered by the photoresist coating. Typically,this can be implantation of a dopant, deposition of another material onthe substrate or an etching of the substrate. This is usually followedby the removal of the resist coating from the substrate typically by anoxygen plasma etch or a wet solvent strip.

This invention is explained below in further detail with references toexamples, which are not by way of limitation, but by way ofillustration.

Examples 1 and 2 below illustrate the synthesis procedure involved ingenerating the acetal polymers.

EXAMPLE 1

Synthesis of partially cross-linked Poly[p-(2-ethyl hexyloxy ethoxy)styrene/p-hydroxystyrene]

A polymer solution was prepared by dissolving 90 grams of poly(p-hydroxystyrene) (PHS) in 310 grams of propylene glycol monomethylether acetate (PGMEA) at room temperature. The weight average molecularweight of the PHS was 8000 and the polydispersity (PD) was 1.17. Thesolution was concentrated by distilling at 60 to 70° to form a 30% solidsolution. To this concentrated solution, 1.47 grams ofcyclcohexanedimethanol divinyl ether, 29.12 grams of 2-ethyl hexyl vinylether (EHVE) and 120 milligrams of oxalic acid was added and stirred atroom temperature for about 16 to 24 hours. After the reaction, 4 gramsof pyridine was added and stirred for 2 hours. The solution was thenwashed with a solution of hexane/acetone/water. The PGMEA layer wasseparated and distilled to remove the residual hexane/acetone/water. Thepolymer was found to have 20-22% blocking levels of EHVE groups by ¹³C-NMR. The molecular weight of the polymer was approximately 34000 andthe PD was approximately 1.73.

EXAMPLE 2

Synthesis of partially cross-linked Poly[p-(cyclohexyloxy ethoxy)styrene/p-hydroxystyrene]

A polymer solution was prepared by dissolving 60 grams of poly(p-hydroxystyrene) (PHS) in 310 grams of propylene glycol monomethylether acetate (PGMEA) at room temperature. The weight average molecularweight of the PHS was 8000 and the (PD) was 1.17. The solution wasconcentrated by distilling at 60 to 70° to form a 30% solid solution. Tothis concentrated solution, 1.47 grams of cyclcohexanedimethanol divinylether, 15.75 grams of cycolhexyl vinyl ether (CHVE) and 90 milligrams ofoxalic acid was added and stirred at room temperature for about 16 to 24hours. After the reaction, 4 grams of pyridine was added and stirred for2 hours. The solution was then washed with a solution ofhexane/acetone/water. The PGMEA layer was separated and distilled toremove the residual hexane/acetone/water. The polymer was found to have20-22% blocking levels of CHVE groups by ¹³ C-NMR. The molecular weightof the final polymer was approximately 34000 and the PD wasapproximately 1.73.

EXAMPLES 3 AND 4

Formulation and lithographic procedure for the polymers synthesized inExamples 1 & 2.

The polymer synthesized is example 1 and 2 (96.875% by weight) were bothformulated with 3% by weight of a triphenylsulphonium salt (photoacidgenerator), and 0.125% by weight of base additives. The concentrationsof the above components are based on the percentage of total solids. Theabove components were dissolved in PGMEA to form a 16% by weight solidsolution.

After the formulation, the resist solutions were filtered through a 0.2um filter and used directly for lithography. The wafers were then spunto give a uniform film thickness of around 7800 Å. These photoresistcoated wafers were then soft baked at 115° C. for 60 seconds. The softbaked photoresist coated wafers were then exposed to 248 nm wavelengthlight on an ISI XLS 7800 0.53 NA stepper. The exposure dose was 15mj/cm². After completion of exposure, the wafers were subjected to apost exposure bake (PEB) at 100° C. for 60 seconds. Following the PEB,the wafers were puddle or spray-developed using a 0.26N tetramethylammonium hydroxide aqueous developer.

Each imaged photoresist-coated substrate was evaluated for severalimportant properties, such as standing waves, % film shrinkage and equalline/space pair resolution (res.). The % film shrinkage, which is ameasure of the volatility of the acid-labile by-products, was calculatedby measuring the thickness before exposure T1, and the thickness of thefilm after exposure T2. The % film shrinkage is the difference inthickness before and after the exposure and is calculated by the formula((T1-T2)/T1) ×100. The results are summarized in Table II.

                  TABLE II                                                        ______________________________________                                                        Exposure                                                      Ex-   Polymer   dose     Resolution                                                                            Standing                                                                             % Film                                ample Synthesized                                                                             (mJ/cm.sup.2)                                                                          μm   wave   Shrinkage                             ______________________________________                                        5     Example 1 15       0.175-0.200                                                                           Ac-    0.60                                                                   ceptable                                     6     Example 2 15       0.175-0.200                                                                           Ac-    0.78                                                                   ceptable                                     ______________________________________                                    

The results show that the photoresist compositions have excellentresolution of 0.2 μm and below with good sensitivity. The compositionsalso have good film shrinkage and standing wave properties.

EXAMPLE 5

Effect of cross-linking on resist profiles and thermal flow

To determine the effect of cross-linking on resist profiles and thermalflow properties, two acetal-containing polymer samples were synthesizedwith and without cross-links. Both polymers contained nearly identicalmole% of EHVE and t-boc protecting groups. The polymer withoutcross-linking had a molecular weight of 8000, while the partiallycross-linked polymer has a molecular weight of approximately 34,000. Thetwo polymers were synthesized under conditions similar to examples 1 and2, except the non-cross-linked polymer synthesis did not contain thedivinyl ether. Both polymers were also formulated and lithographicallyprocessed under the same conditions as examples 3 and 4. After resistexposure and development, a SEM analysis showed similar resolution, butthe profiles of the partially cross-linked polymer were more vertical.

In addition, the thermal stability of the resist with partiallycross-linked polymer was better than the resist without cross-linking.This was determined by baking both resists at various temperatures for 4minutes. The non-cross-linked polymer resist profiles started to degradeand became less vertical (more rounded) at 120° C. In contrast, thepartially cross-linked polymer resist profile did not start to degradeuntil 130° C. The 10° C. change in thermal flow is a significantimprovement in terms of process temperature window performance.

The foregoing is illustrative of the present invention and is notconstrued as limiting thereof. The invention is defined by the followingclaims with equivalents of the claims to be included therein.

What is claimed is:
 1. A process for preparing an organically solublepartially cross-linked polymer comprising the steps of: providing areactant polymer with one or more monomer units, wherein at least one ofthe monomer units contain 1 or more pendant COOH groups or hydroxylgroups; and reacting said polymer with a polyvinyl ether in the presenceof an acid catalyst to form links between at least two polymer chains.2. The process of claim 2 wherein the polyvinyl ether has the formula:##STR13## wherein Y is a polyvalent radical; R¹, R² and R¹⁹ are eachindependently selected from a hydrogen, a linear or branched C₁ to C₁₈alkyl or haloalkyl, a C₃ to C₁₈ cycloalkyl, a C₆ to C₁₄ aromatic, a C₆to C₃₀ alkaryl or a C₆ to C₃₀ aralkyl; and n is an integer of 2 or more.3. The process of claim 2 wherein Y is a linear or branched C₁ to C₁₈alkyl, haloalkyl or alkoxy, a C₆ to C₁₀ cycloalkyl, a C₆ to C₃₀aromatic, a C₆ to C₃₀ alkaryl or a C₆ to C₃₀ aralkyl.
 4. The process ofclaim 3 wherein said reactant polymer is a phenolic-based polymer, ahydroxy cycloalkyl-based polymer or mixtures thereof.
 5. The process ofclaim 4 wherein said reactant polymer is a hydroxystyrene based polymer.6. The process of claim 1 further comprising reacting a monovinyl etherof the formula: ##STR14## with said reactant polymer to partiallysubstitute the hydroxyl groups of said polymer with acid labileprotecting groups; wherein R³, R⁴ and R²⁰ are each independentlyselected from a hydrogen, a linear or branched C₁ to C₁₈ alkyl orhaloalkyl, a C₃ to C₁₈ cycloalkyl, a C₆ to C₁₄ aromatic, a C₆ to C₃₀alkaryl or a C₆ to C₃₀ aralkyl; ; and R⁵ is a linear or branched C₁ toC₁₈ alkyl or haloalkyl, a C₃ to C₁₈ cycloalkyl, a C₆ to C₁₄ aromatic, aC₆ to C₃₀ alkaryl, a C₆ to C₃₀ aralkyl; or a linear, branched, cyclicaromatic or olefinic group.
 7. The process of claim 6 wherein saidreactant polymer has at least one monomer unit of the formula: ##STR15##wherein X is defined as: ##STR16## or mixtures thereof; and wherein R⁶,R⁸ and R⁹ are each independently a hydrogen, a C₁ to C₄ alkyl, ahalogen, a nitro, a cyano, or a combination thereof.
 8. The process ofclaim 7 wherein said reactant polymer further comprises having at leastone monomer unit of the formula: ##STR17## wherein R¹⁰ is a hydrogen, aC₁ to C₄ alkyl, a halogen, a nitro, a cyano or a combination thereof;R¹¹ is a hydrogen, a linear or branched C₁ to C₁₈ alkyl or haloalkyl, aC₃ to C₁₈ cycloalkyl, a C₆ to C₁₄ aromatic, a C₆ to C₃₀ alkaryl or a C₆to C₃₀ aralkyl; X¹ is ##STR18## and R¹⁷ and R¹⁸ are each independently ahydrogen, a C₁ to C₄ alkyl, a halogen, a nitro, a cyano, or acombination thereof.
 9. The process of claim 7 wherein said reactantpolymer further comprises having a monomer unit of the formula:##STR19## wherein R¹² is a hydrogen, methyl or ethyl group, or a grouphaving the formula --CH₂ --COOR⁷ ; R⁷ is a primary, secondary andtertiary carbon attached to an alkyl or aromatic group, R¹³ is a bond ormethylene, and R¹⁴ is a primary, secondary or tertiary carbon attachedto an alkyl or aromatic group.
 10. The process of claim 7 wherein saidreactant polymer is reacted with at least one di-tertiary-butyldicarbonate and tertiary-butyl haloacetate in the presence of an organicor inorganic base to form t-butoxy carbonyl and/ort-butoxycarbonylmethyl groups.
 11. The process of claim 7 wherein thepolyvinyl ether is selected from the group consisting of:cyclohexanedimethanol divinyl ether, ethylene glycol divinyl ether,butanediol divinyl ether, hexanedimethanol divinyl ether,diethyleneglycol divinyl ether, triethyleneglycol divinyl ether,tetraethyleneglycol divinyl ether, trimethoyl propane trivinyl ether,erithritol tetravinyl ether, poly-ethylene oxide divinyl ether andpolybutylene oxide divinyl ether.
 12. The process of claim 1 whereinsaid acid catalyst is selected from the group consisting of:hydrochloric acid, sulfuric acid, oxalic acid, benzoic acid,p-nitrobenzoic acid, malonic acid, para-toluene sulfonic acid, andpyridinium-para-toluene sulfonate.
 13. The process of claim 7 wherein R⁵contains 6 or more carbon atoms.
 14. The process of claim 7 wherein saidmonovinyl ether is selected from the group consisting of: ethyl hexylvinyl ether, cyclohexyl vinyl ether, tertiary butyl vinyl ether anddodecyl vinyl ether.
 15. The process of claim 14 wherein the polyvinylether is cyclohexanedimethanol divinyl ether, and the monovinyl ether isethyl hexyl vinyl ether.